1. Field of the Invention
The invention relates to the field of ceramics and concerns a ceramic network, such as can be used, for example, as a deep-bed filter, more particularly as a molten metal filter, as a support for filtration, heat exchanger, regenerator, electrically heatable thermostat, catalyst support, burner element for radiant heaters and space heaters, reaction chamber fill element, sound absorber, stiffening element for panels, or as a ceramic reinforcement material for metal matrix composites (MMC""s), and a method for the production and utilization thereof.
2. Discussion of Background Information
Ceramic networks in the form of open-cell ceramic foams are known.
Methods are known for the manufacture of such open-cell ceramic foams using the so-called xe2x80x9cSchwartzwalder method,xe2x80x9d which is used industrially and is the most common. This method is described in U.S. Pat. No. 3,090,094. In accordance with this method, the desired component is cut from an open-cell polymer foam and subsequently impregnated with a suspension of ceramic particles and water or solvent. Then the impregnated polymer foam is mechanically squeezed one or more times, and subsequently dried. Next the polymer foam is burned out, followed by sintering of the remaining ceramic coating.
Open-cell ceramic foam manufactured by this method is a replication of the cell-like polymer structure of the starting material. As a result of burning out the polymer foam, the remaining ceramic struts are hollow. The cross-section of these struts is three-cornered, and the shape of the cavities is also three-cornered in cross-section. The ceramic coating is often cracked at the edges of the cavities. The cavities and the cracks result in a very low mechanical strength. Because the susceptibility to cracking is increased even further by shrinkage of the ceramic coating during sintering, relatively low-shrink materials are used, but they exhibit high internal porosity after sintering. This likewise results in low mechanical strength. See J. Am. Ceram. Soc. 77(6), 1467-72 (1994).
Thus, the ceramic foams manufactured from polymer foams with the aforementioned method have cavities with a concave, three-cornered cross-section inside the ceramic struts. See (Cahn, R. W., Haasen, P., Kramer, E. J. (ed.): Material Science and Technology, Vol. 11, VCH 1994, p. 474. The shape of this cavity is very unfavorable for the mechanical strength of the struts in the ceramic foam, since the load-bearing area of the points of the triangles is only very small. Due to the susceptibility of the brittle ceramic to the formation of cracks, the very sharply pointed shape of the three-cornered cavities is also problematic, since cracks nearly always form starting from there, further decreasing the strength of the ceramic strus. See (J. Am. Ceram. Soc. 77(6), 1467-72 (1994). Consequently, the foams produced with the Schwartzwalder method have a low mechanical strength, which is disadvantageous for the aforementioned applications as well as for the handling and transport of such ceramic foams.
The foam materials used for molding are produced by foaming a mixture of various chemical components. During the reaction of the fluid components with one another, a gas is produced, which causes gas bubbles to form and grow in the fluid. Moreover, the starting components polymerize, increasing the viscosity of the fluid. At the end of the reaction, a solid polymer forms that contains a large number of gas bubbles (polymer foam). The size of the bubbles in the polymer foam can be controlled within certain limits by the choice of the starting components and by regulating the reaction.
By a subsequent treatment known as reticulation, the membranes separating the gas bubbles are completely removed by chemical or thermal means, creating the open-celled polymer foam required for manufacture of the ceramic. This foam now consists only of polymer struts that have formed between three adjacent gas bubbles. See Klemper D. and Frisch K. C. (Ed.): Handbook of Polymeric Foams and Foam Technology, Hanser 1991, p. 24.
As a result of the nature of gas bubble foaming, the surfaces of the polymer foam are always concave in shape. Thus, the cross-sections of the polymer struts forming the foam have the shape of triangles with concave sides having very sharply angled points. See Klemper D. and Frisch K. C. (Ed.): Handbook of Polymeric Foams and Foam Technology, Hanser 1991, p. 28/29. This is considered a law of nature for all foamed materials.
Also, the gas bubbles that occur during foaming of the polymers cannot be created in unlimited size. When the gas bubbles are too large, the foam collapses before polymerization has brought about solidification of the foam. See Klemper D. and Frisch K. C. (Ed.): Handbook of Polymeric Foams and Foam Technology, Hanser 1991, p. 9. The upper limit for the most commonly used polymer foam of polyurethane flexible foam is approximately 5 pores per inch (approximately 5 mm maximum cell size). Hence this also presents a limitation on the possibilities for using polymer foam for the manufacture of ceramic foam.
It is further known that the foam used is generally polyurethane foam. See (Am. Ceram. Soc. Bull. 71 (11) (1992). However, a disadvantage of the use of polyurethane as the starting structure for ceramic foam manufacture is that gases which are toxic or hazardous to health, e.g., isocyanates or hydrogen cyanide, can be released during the necessary thermal decomposition of the polyurethane. See J. Polymn. Sci. C, 23, pp. 117-125 (1968)
To somewhat mitigate the problems of mechanical strength, DE 35 40 449 and DE 35 39 522 propose applying multiple coatings to the polyurethane foam used. This increases the thickness of the ceramic struts and thus the mechanical strength of the sintered ceramic foam as well.
The increased process cost for the multiple coating is problematic. Furthermore, the ceramic coating has only low strength prior to sintering, and consequently the mechanical loading of the coated polymer foam necessary for separating the excess suspension during multiple coating frequently leads to new defects in the coating. In principle, however, multiple coating also does not eliminate the disadvantage mentioned of unfavorably shaped concave three-cornered cavities of the struts.
It is likewise known to use ceramic fibers as monofilaments or multifilaments for the manufacture of porous ceramics: which fibers can be laid, knitted, sewn or glued. See IChemE Symposium Series No. 99, pp 421-443 (1986); MTZ Motortechnische Zeitschrift 56(2), pp 88-94 (1995).
A disadvantage here is that such ceramic fibers are difficult and expensive to produce, and thus are very expensive, and are difficult to process since they are very brittle. For example, knitting techniques can be used only to a limited degree here. Hence, only a limited selection of ceramic materials may be used for such fibers, which makes it difficult or next to impossible to modify the properties of the porous ceramic produced therefrom. Moreover, such porous structures are flexible since the fibers are not joined to one another at the contact points. This is disadvantageous in the case of filtration or mechanical loads, since these ceramics are not very stiff overall and, in addition, fiber abrasion is produced, especially with multifilaments.
Joining of such fibers can also be undertaken, see U.S. Pat. No. 5,075,160; although this is only of interest for the typical applications if ceramic joining is created. This, too, is difficult and expensive to achieve, generally using CVD or CVI techniques, but the choice of materials is again very limited.
In addition, it is known to manufacture open-pored materials from polymer fibers, natural fibers, or carbon fibers, and then to convert them directly to a ceramic material, e.g., by pyrolysis or with the addition of other chemical elements through the fluid or gas phase and reaction of the fibers with these elements. However, the conversion of these starting fibers to open-pored ceramics is complicated and can only be controlled by costly methods; this severely limits the choice of materials and geometries.
The present invention relates to a ceramic network and a method for the production thereof in which the mechanical strength is improved and/or an application-specific structure of the ceramic network can be specifically controlled.
The present invention provides for a method for producing a ceramic network from a fiber network comprising fiber selected from polymer fiber, natural fiber or mixtures thereof comprising the steps of impregnating the fiber network with a ceramic suspension, removing excess ceramic suspension from the impregnated fiber network, drying the impregnated fiber network, removing the fiber network, and sintering the remaining network to form the ceramic network.
The fiber can have a cross-sectional area having a circular, a substantially circular, a convex or a multiply convex outline.
The fiber network can be impregnated one or more times with a ceramic suspension.
The fiber network can be a structured or an unstructured network. The fiber network may be a structured network having a uniform structure, a repetitive structure, or a combination thereof The structured fiber network can have a directionally dependent structure.
The fiber network can be a two-dimensional connection of the fibers, a three dimensional connection of the fibers or combination thereof and the fibers can be connected by gluing, bonding, plaiting, felting, weaving, knitting, embroidering, sewing, embossing or a combination thereof.
The fiber can be selected from polyester, polyethylene, polyamide, cotton, cellulose, coco, jute, hemp, flax, horsehair, or mixtures thereof.
The fiber can be coated with a material other than ceramic or the fiber can be uncoated.
The fiber network can be removed by etching the fiber network, dissolving the fiber network or bacteriologically removing the fiber network. The fiber network can be removed by burning out the fiber network by increasing temperature in a linear or nonlinear fashion or in stages. Burning out the fiber network can take place in air, a reducing atmosphere, an oxidizing atmosphere or an inert atmosphere. The burning out of the fiber network can be completely accomplished or can be substantially free of residue.
The burning out of the fiber network can take place by increasing temperature in stages and during which substantially residue-free burnout occurs in the first or one of the first temperature stages.
The fiber network can have completely, substantially or partially the shape and/or structure of a component to be manufactured from the ceramic network.
The present invention also provides for a ceramic network comprising struts being joined together two-dimensionally or three dimensionally, having a material-consistent connection to one another and having an interconnected cavity over their points of contact, the cavity having a cross-sectional area with a circular, substantially circular, a convex, or multiply convex outline.
The ceramic network can have a two-dimensional or three dimensional structure shaped differently depending upon a direction of fluid flow through the ceramic network.
The two-dimensional or three dimensional structure of the network can be uniform, repeating or a combination thereof.
The two-dimensional or three dimensional stricture of the network can be directionally dependent and uniform, repeating or a combination thereof.
The ceramic network can have a shape and/or structure which corresponds entirely, substantially or in part with the shape or the structure of a component to be manufactured.
The ceramic network has many utilities. The ceramic network may be used as a ceramic filter for contacting fluids, as a ceramic filter for contacting molten material or as a molten metal filter. The ceramic network may be used as a deep-bed filter, a support for filtration, a heat exchanger or as a heat regenerator.
The ceramic network may be used as a catalyst support, as a reaction chamber fill element, as a burner element for radiant heaters or for space heaters, as a heat element for a thermostat, as a control element for a thermostat, or as a burner element for space heaters.
The ceramic network may be used for supporting a mechanical load, as a sound damping element, as a stiffening element for lightweight building elements, as a mirror support, as a thermal protective tile, as a ceramic reinforcement material for metal matrix composites, as a burner element for space heaters or as a ceramic reinforcement material for light metal alloy.
The ceramic network may be used as a brake material, as an abrasive or as a carrier for an abrasive.
Using the solution according to the present invention, a two-dimensional or three-dimensional ceramic network is obtained whose ceramic struts have cavities with a cross-sectional area having a circular or nearly circular or largely circular or a convex or multiply convex outline. In this way, a uniform coating thickness, e.g., a uniform load-bearing area, is achieved. Furthermore, formation of cracks can largely be avoided, so that the ceramics according to the present invention and manufactured in accordance with the invention demonstrate greater mechanical strength.
For some applications of open-pored ceramic networks, e.g., for use as a filter material, regenerator, or sound damper, it is important for the structure of the cast foams to be nearly the same in each direction in space. This is achievable with the open-pored foam ceramics known from the prior art, but can likewise be achieved with the ceramic network in accordance with the present invention.
For some applications, however, it is a great advantage if the ceramic network can be constructed with a specific structure in one or two or all three directions in space. The structure can be uniform and/or repeated in an advantageous manner here. This means that a uniform, directionally dependent structurexe2x80x94with as many repeats as desiredxe2x80x94of the fiber network, for example, a woven cotton fabric with a woven-in pattern, can produce a ceramic network with exactly the same structure.
In the case of the flowing-through of gas or fluids, it can be advantageous to produce a predominant direction or, in the case of use as a reinforcement for metals, to achieve a predominant direction of mechanical strength. Structuring of the ceramic network in such a way is not known and cannot be achieved in the prior art, but can be produced easily with the method in accordance with the present invention.
For example, a knitted fabric or knit can be produced such that flow through it in one direction in space is nearly unhindered. A ceramic network can then be produced without difficulty from this knitted fabric or knit using the method in accordance with the present invention.
Another advantage of the solution according to the present invention is that relatively large cell sizes can be produced with the ceramic network in accordance with the present invention.
In the prior art methods, the polymer foams used as a basis for the open-cell ceramic foams can only foam to a certain level. When this point is exceeded, the open-celled structure of the foams is destroyed more and more and the foam finally collapses. The maximum achievable cell size is approximately 5 mm.
Using the method in accordance with the present invention, ceramic networks with far larger cell sizes can be produced easily.
In addition, it is advantageous if the starting structure is manufactured of polymer fibers and/or natural fibers or polymer and/or natural fiber bundles, because harmless decomposition products which are not toxic or hazardous to the health are created when the fibers or fiber bundles are removed or burned out.
The increase in temperature when burning out the fiber network can be chosen such that it takes place in linear or nonlinear fashion or in stages. All possible atmospheres may be present during the process. Burnout takes place until the fiber network is burned out completely or nearly completely free of residue.
When temperature increase in stages is chosen for this purpose, it is advantageous if the burnout occurs in the first or one of the first temperature stages.
The present invention provides a method for producing a ceramic network in which a material-consistent connection exists among the individual ceramic struts. Material-consistent here means that the struts and the connection between the struts are made of the same material.
Moreover, in the ceramic network according to the present invention, there exists an interconnection among the cavities in the struts and, in particular, interconnected cavities are also present at the points of contact between the struts. This is produced by the manufacturing process in accordance with the present invention in which a contact point between two fibers, for example, is enveloped as a whole by the ceramic suspension and, after both fibers have been burned out, the ceramic struts are continuous and also have a continuous cavity at the points of contact.
In the method according to the present invention, fibers that are uncoated or completely uncoated with ceramic material are used advantageously.
It is also possible here for a fiber bundle, for example, to be surrounded by an enveloping enclosure.
It is also advantageous for a network to be designed such that it completely or essentially completely or partially corresponds, with regard to its shape and/or structure, to the shape and/or structure of the component to be manufactured.
To this end, a fiber network is manufactured which completely or essentially completely or partially has the shape and/or structure of the desired component to be manufactured. This fiber network is then treated in accordance with the present invention and the desired ceramic network is created.
During manufacture of the ceramic network, reduction in size as compared to the fiber network may take place as a result of shrinkage, even though shape and/or structure still may correspond to that of the desired component.
The present invention is explained in greater detail below with several exemplary embodiments.